This happens when no calendar was connected at the time the account was set up. Demodesk chooses the default location for your meeting types once, when they are first created. With a Google account connected it chooses Google Meet, with a Microsoft account it chooses Microsoft Teams, and with neither it falls back to Custom with an empty value. An empty Custom location is what produces an invite with no join link.
Connecting a calendar afterwards does not fix it, because that default is only ever chosen once. Open each meeting type and set the location to Google Meet or Microsoft Teams.
Meetings that are already in the calendar keep the empty location. Those have to be rebooked if the join link matters.